1. Field of the Invention
The present invention relates to a digital demodulation apparatus for digitally demodulating a signal, more particularly to an orthogonal frequency division multiplex (OFDM) demodulation apparatus for demodulating an OFDM signal having 0 as the amplitude of a specific frequency component.
2. Description of the Related Art
When transmitting digital signals, there are known the methods of phase modulation (PM) or amplitude modulation (AM) of a single frequency carrier wave signal based on a digital signal.
As a specific example of such modulation methods, there are known phase shift keying (PSK) for changing only the phase of the carrier wave signal and quadrature amplitude modulation (QAM) for changing both the phase and the amplitude of the carrier wave signal.
In the above modulation methods, the single frequency carrier wave signal is modulated so as to have an occupied bandwidth of an extent fitting in the transmission band.
Recently, as a new modulation method, proposal has been made of the modulation method known as the orthogonal frequency division multiplex (OFDM) modulation method.
In OFDM modulation, a plurality of orthogonal carrier wave signals are generated in the transmission band to divide the transmission band and the carrier wave signals are each subjected to PSK processing or QAM processing by digital signals.
Since OFDM modulation divides the transmission band by a plurality of carrier wave signals, the band per carrier wave signal becomes narrow and the modulation time per carrier wave signal becomes long. When the transmission band is the same, however, the overall transmission time obtained as a result of the modulation of each of the plurality of the carrier wave signals is no different from the modulation methods used widely previously, for example, the PSK and QAM methods.
In OFDM modulation, a plurality of carrier wave signals are transmitted in parallel, so the transmission rate per digital data to be transmitted, that is, symbol, becomes slower. In a transmission path with so-called multiple path interference, it is possible to reduce the relative delay time of the multiple path interference wave with respect to the time length for symbols. As a result, OFDM modulated signals are resistant to the effects of multiple path interference. Application of OFDM modulation to transmission of digital signals by earth waves taking advantage of this point is drawing great interest.
For signal processing in OFDM modulation, it is necessary to perform inverse discrete Fourier transformation (IDFT) at a high speed, while for signal processing in OFDM demodulation, it is necessary to perform discrete Fourier transformation (DFT) at a high speed.
In the past, it was difficult to realize such signal processing inexpensively and at a high speed, but recent advances in electronics technology and semiconductor technology have made possible the provision of semiconductor devices which can perform DFT and IDFT efficiently by hardware-like processing and electronic circuits using the same. Accordingly, it has become possible to perform OFDM modulation, or OFDM demodulation, using such semiconductor devices and electronic circuits.
The characterizing feature of OFDM modulation is that orthogonal carrier wave signals are generated for each predetermined bandwidth obtained by dividing a transmission channel (transmission band), but that the OFDM modulated signals are digital signals of a low data transmission rate which can fit in their respective bandwidths and that the individual carrier wave signals are not modulated by digital signals, but all carrier wave signals are modulated all at once by IDFT processing.
A summary of the OFDM modulation will be given below.
On the transmission side performing the OFDM modulation, waveforms of carrier wave signals corresponding to the values "1" or "0" of the digital data to be transmitted are defined for the plurality of carrier wave signals #1 to #n. By adding up and combining the carrier wave signals #1 to #n showing the values of the digital data to be transmitted, OFDM modulated signals are obtained. That is, if these carrier wave signals #1 to #n are arranged in order on the frequency axis and the amplitude and phase of the carrier wave signals are defined by the digital data to be transmitted, it is possible to define the waveform of the data (symbols) of the OFDM modulated signals in a predetermined time length by the digital data to be transmitted.
At the receiving side which demodulates the OFDM modulated signals, the OFDM modulated signals are received and the waveforms of the carrier wave signals are discriminated to establish correspondence with digital data, whereby it is possible to demodulate the OFDM modulated digital data sent from the transmission side for each carrier wave signal.
In OFDM modulation, it is possible to transmit the digital data by defining the carrier wave signals by two phase states and modulating them by the binary PSK (BPSK) method and also possible to define a large number of phases and amplitudes and transmit the data in multilevels.
Digital data given multiple values for each of a plurality of carrier wave signals is transmitted by defining the amplitude and phase of the carrier wave signals and obtaining the waveforms of the same. The processing operation for obtaining the waveforms is the so-called IDFT processing. Therefore, in OFDM modulation, it is possible to obtain an OFDM modulated signal using an IDFT circuit.
Conversely, the OFDM modulated signals obtained by IDFT processing in this way can be demodulated by DFT processing on the receiving side.
However, in a transmission path with multiple path interference, there is the problem that it is difficult to accurately detect the synchronization symbol duration due to the leakage of signals into the synchronization symbol duration or noise entering into the synchronization symbol duration in a transmission path with noise.
Further, when viewed from the perspective of transmission of data, there is the problem of deterioration of the transmission efficiency due to insertion of the synchronization symbol. From this viewpoint, it is desired to reduce to an extreme the number of the synchronization symbols.
On the other hand, from the viewpoint of the stable operation of the PLL circuit performing the reproduction of the clocks and the DFT circuit performing demodulation of the received signals, there is the problem that it is not desirable to reduce the number of the synchronization symbols since reduction of the synchronization signals means a reduction of the reference signals.